Previously, an electron emission devices were made using a spindt-type electron emission source. Such an electron emission source comprises layers laminated with a material such as Mo, Si, etc. and processed to a sharp tip. However, this spindt-type electron emission source has an ultra-fine structure and its manufacturing method is very complicated, requiring a high degree of precision. Consequently, manufacturing large field emission display devices according to this method is extremely difficult. The electron emission device is preferably a field emitter array (FEA) type electron emission device.
To solve this problem, a carbon-based material having a low work function has recently been proposed for use as the electron emission source. In particular, carbon nanotubes (CNT) perform well as electron emission sources because they have high aspect ratios and small tip radii of curvature of about 100 Å. Therefore, electrons are readily emitted by application of a low external voltage, e.g. a voltage as low as about 1 to about 3 V/μm.
Use of carbon nanotubes having low work functions as the electron emission source enables low voltage operation and easy manufacturing of the electron emission source. These advantages enable the manufacture of large field emission displays.
Generally, such an electron emission source is fabricated by first forming the carbon nanotubes into a paste along with a solvent, a resin, etc. The paste is then screen printed between two substrates and fired.
The carbon nanotubes may be synthesized according to several different methods. For example, physical methods of synthesis may include electric discharge or laser deposition. Chemical methods of synthesis may include screen printing or chemical vapor deposition.
The carbon nanotubes are primarily synthesized using the electric discharge method. FIG. 1 is a schematic depicting an apparatus performing the electric discharge method. According to this method, a direct current source is applied between a cathode 10 and an anode 20. Both the anode and cathode each comprise a graphite or metal rod. A discharge occurs between the anode and cathode generating numerous electrons which then collide with the anode rod 20. The collision of electrons with the anode rod causes carbon clusters to become disjoined from the anode rod. These carbon clusters are then condensed onto the surface of the cathode rod, which is maintained at a low temperature, thereby creating carbon nanotubes.
According to the laser deposition synthesis method, a graphite rod is placed in a high temperature reaction furnace and is irradiated by a laser beam. This irradiation evaporates the graphite, and the evaporated graphite is then adsorbed on a collector that is maintained at a low temperature.
To fabricate either single-walled or multi-walled carbon nanotubes using physical synthesis methods such as electric discharge and laser deposition, a cavity must be formed in a pure graphite anode rod and filled only with catalytic metals such as Y, Ni, Fe, Co, etc.
According to the screen printing method, a cathode layer is fabricated by screen printing a paste comprising mainly single-walled carbon nanotubes formed by arc discharge. The cathode layer is then used as the CNT for the electron emission device.
According to the chemical vapor deposition (CVD) method, carbon nanotubes are synthesized by applying a carbon source gas such as acetylene to formatted fine catalytic particles.